1. Field of the Invention
The field of the invention is generally hydrophones and in particular is neutral density, shock-resistant hydrophones.
2. Description of the Prior Art
A hydrophone is an underwater transducer for converting underwater acoustic signals to electrical signals. Although hydrophones can generally be used as sound generators as well as detectors, this application will primarily discuss hydrophones for underwater sound detection.
Most underwater acoustic detectors have been designed using a sensor element of a piezoelectric ceramic such as lead-zirconate, lead-titanate or barium-titanate. The piezoelectric characteristics of these materials provide for the conversion of acoustic energy to electrical energy through the compression of the piezoelectric material. Although the above mentioned materials are efficient for the acoustic-electrical conversion, they suffer from several problems.
These ceramic materials are brittle and cannot sustain large tensile strains without breaking. This fragility presents severe problems when the hydrophone is expected to survive the explosive shock pressures associated with submarine warfare or seismic exploration. Several methods have been used to prestress ceramic sensors in order to decrease the possibility of tensile failure of the ceramic. Representative methods are wrapping the sensor with fiber glass and using stress rods to keep the sensor material in compression even at the highest tensile loads. However all these methods tend to degrade the acoustic performance of the hydrophone.
Another disadvantage inherent in ceramic hydrophones arises from the density of the piezoelectric ceramic. The density of ceramics is typically in the range of 7.0 to 7.7.times.10.sup.3 kg/m.sup.3 compared to a density of water near 1.times.10.sup.3 kg/m.sup.3. Note that the density of water varies a few percent dependent upon temperature and salinity. This mismatch of densities presents design problems in constructing neutrally buoyant hydrophones.
A further disadvantage of ceramic sensors arises from the high velocity of sound in the ceramic and the high mechanical Q of ceramic. Because of these properties, ceramic sensors are generally unsuitable for broadband, high-frequency applications.